I frequently get asked by various poeple to fix or help them with their computers (Windows XP, Windows 7, OSX Leopard, OSX Snow Leopard, OSX Lion). What is currently the best secure, free (or cheap) and most imporantly simple way to remotely access the desktop of another Mac or PC over the internet from my Mac (OSX Lion).

It should be especially simple for the person at the other end asking for help because often they are not too computer-savvy. It should invlove nothing more than starting an application or enabling a checkbox in the settings. It would also be really nice if I could do remote access from my iPhone while I am on the go. It should also be possible to exchange files.